The recent developments in aluminum-lithium (Al-Li) alloys are of great interest to the aerospace community because of the pronounced effect of lithium on simultaneously decreasing the density and increasing the stiffness of aluminum.
Al-Li alloys produced by conventional casting methods, such as direct chill (DC) casting, are limited to lithium levels of no greater than about 2.5 wt. %. Above this amount, difficulties are encountered in producing sound, high quality ingots that do not contain coarse second phase particles along grain boundaries and which have sufficiently low levels of embrittling hydrogen and alkali metal impurities.
The primary phase responsible for strengthening binary Al-Li alloys is the ordered metastable phase, .delta.'(Al.sub.3 Li). At temperatures below its well defined solvus line, .delta.' is in metastable equilibrium with the aluminum matrix. At temperatures above its solvus line, the equilibrium .delta. phase (AlLi) is formed.
Zirconium is typically added to aluminum alloys in order to control grain size and retard recrystallization. Zirconium reacts with aluminum to form Al.sub.3 Zr which, depending upon zirconium concentration and cooling rate, can have either a metastable cubic or equilibrium tetragonal crystal structure. However, only the cubic phase, which forms as fine, spherical particles is effective in controlling grain size and retarding recrystallization. Metastable cubic Al.sub.3 Zr has the Ll.sub.2 crystal structure and is isomorphous with the primary strengthening phase in Al-Li alloys, .delta.'. Cubic Al.sub.3 Zr acts as a preferred site for precipitation of .delta.' in Al-Li alloys, but unlike .delta.', is highly resistant to dislocation shear. In sufficient quantity, cubic Al.sub.3 Zr reduces the tendency for planar slip in Al-Li alloys thereby improving alloy strength as well as ductility. In DC cast alloys, the maximum amount of zirconium that can typically be added is 0.13 wt. %. Beyond this level, large, needle shaped particles of tetragonal Al.sub.3 Zr, which do not have any microstructural benefit, are formed instead.
It is recognized in the art that alloy production methods with cooling rates greater than that of DC casting can be used to refine grain size, suppress the formation of large second phase particles along grain boundaries, increase the amount of zirconium that can be added to an alloy without formation of tetragonal Al.sub.3 Zr, and reduce hydrogen and sodium levels in the end product. One such solidification technique, is rapid solidification processing (RSP).
In accordance with the typical RSP method, the alloy is rapidly solidified from the melt into either powders or continuous ribbons (which are subsequently comminuted into powder form). The powders are then consolidated into bulk compacts. The consolidation step involves one or more conventional powder metallurgy processing techniques including, direct powder rolling, vacuum hot compaction, forging, extrusion, etc.
A disadvantage of RSP methods, especially as applied to the production of Al-Li alloys is that, complex Al-Li oxides which form quickly on the surface of rapidly solidified powders, are often retained in the consolidated product as continuous stringers or as a semi-continuous network along prior particle boundaries. The oxides act as preferred sites for crack initiation and propagation resulting in an alloy with poor ductility and fracture toughness. Also, because of the hydrated nature of the Al-Li oxide films, the hydrogen level of the alloy can be adversely increased. U.S. Pat. No. 4,661,172 issued to Skinner, et al. discloses a family of low density Al-Li-Cu-Mg-Zr alloys formed by the RSP method. The alloys contain lithium levels ranging between 3.5 and 4.0 wt. % and zirconium levels ranging between 0.2 and 1.5 wt. %. The alloys disclosed by Skinner, et al. exhibit good strength, but have less than optimum ductility and fracture toughness because of the presence of oxides at prior particle boundaries.
In view of the large number of steps typically involved in consolidating rapidly solidified materials, RSP Al-Li alloys are not economically competitive with alloys produced by more direct methods such as DC casting. In addition, the production of billets weighing thousands of pounds, which occurs routinely by DC casting, is extremely difficult, if not impossible, using RSP methods. For these reasons, researchers have turned to alternate methods for production of Al-Li alloys with lithium contents in excess of 2.5 wt. %.
A more economical method for producing Al-Li alloys is a process known as spray casting or spray/brining. The spray casting method is described in detail in U.S. Pat. No. 4,938,275 issued to Leatham, et al.
Unlike RSP, there are no practical limitations restricting the size of billets that can be produced by spray casting. Cooling rates during spray casting are not as rapid as those associated with RSP. However, they are significantly higher than those encountered during DC casting.
Al-Li alloys produced by the spray cast method and having moderately high Li content (i.e., about 2 wt. %) are known from the prior art. For example, U.S. Pat. No. 5,223,216 issued to Lasalle discloses a spray cast Al-Li alloy having the composition Al-2.1Li-1.0Cu-0.4Mg-0.6Zr. Further, published WIPO document No. WO 91/14011 (International Application No.: PCT/GB91/00381) discloses a spray cast Al-Li alloy having the composition Al-2.68Li-1.73Cu-0.86Mg-0.11Zr.
A spray cast Al-Li alloy containing 4 wt. % Li is also known in the prior art. For example, Palmer, Chellman and White ("Evaluation of a Spray Deposited Low Density Al-Li Alloy, ICSF2, Swansea, U.K. September 1993) disclose a medium strength spray cast alloy having the composition Al-4.0Li-0.2Zr. The lithium level of this composition was specifically selected to be close to but less than the maximum solid solubility of lithium in aluminum (approximately 4.2%) in order to achieve the lowest possible density while avoiding the formation of a large amount of the .delta. phase, AlLi, which these authors report is detrimental to ductility and fracture toughness.
Earlier research in the field of RSP Al-Li alloys also suggests that good ductility cannot be achieved in Al-Li alloys containing greater than 4 wt. % Li. See, for example, Meschter, Lederich and O'Neal ("Microstructure and Properties of Rapid Solidification Processed (RSP) Al-4Li and Al-5Li Alloys", Aluminum-Lithium Alloys III, 1986, p. 87). This paper describes an RSP Al-5Li-0.2Zr composition that has been extruded, solution heat treated and peak aged and indicates that the 10 percent minimum volume fraction of .delta. phase which is always present in Al-5Li alloys is twice as high as the generally recognized maximum level below which acceptable ductility and an acceptable strength/ductility ratio are achieved.
As can be seen from the above discussion, the prior art does not teach or suggest spray cast Al-Li alloys which combine both a higher than usual zirconium content (i.e. greater than about 0.13 wt. %) with a lithium content in the 5 wt. % range. Thus, there is a continuing need in the art for a family of ternary (Al-Li-Zr) alloys and method for producing the same which have both a high zirconium content for grain refinement and increased matrix shear resistance and a high lithium content (in excess of 4 wt. %) for density reduction and high stiffness.